Only partialy completed cron and some payments not updating plan renewal dates

[ServWise.com]

I am getting almost every night a problem where the cron is run but only partially completes, this seems to happen if there are more than 1 invoice to issue, if only one then it is okay if more than one is generated it will generate the second invoice but stop there and not complete the sending of the invoice email or the rest of the cron tasks.

 

Also I have had two occasions in the last two weeks where completed payments have not updated hosting plan renewal dates and caused the system to then suspend the accounts even though they had paid. The invoices are marked paid in WHMCS but the renewal dates are not updated.

[WHMCS John]

The cron terminating early before it completes is usually caused by the php memory limit being exceeded. A good guideline figure is to increase it to 2x the max number of invoices you expect generated on any one day.

[ServWise.com]

My PHP memory limit is set to 256MB (per script) I would have expected that to be more than enough, how can I work out the memory needed for generating an invoice?